MattABC

I would love the first available time (1030 or 11 am, I believe). But, I travel with a disabled daughter who has multiple problems, and sometimes it just takes longer to do everything.  We'll probably pick 12 or so, to make sure we can get out of the house on time - 1030 would mean leaving 930-945, & that just might not be possible, especially if she had a bad night or morning.

If you are parking at the port, keep in mind it can fill up.  If you are parking at an off-port facility, check out their shuttle schedule.  For example, hotels might have a fixed schedule you'd want to synchronize with your check-in time.

I would say the earlier you can be onboard drinking a beverage poolside the better off you will be…..

As others have noted, it's a matter of preference. I used to do a later check in to avoid long lines in the cruise terminal, and basically just walked in. But I now try and get an earlier check in time, maybe not the earliest but perhaps an hour to an hour in a half later. The reason being is we choose the first seating in the main dining room and request a table for 2. I already have that confirmed with my travel agent when I initially book a cruise, but I've learned that while Royal says I have a table for 2 in the computer, come sail away time I may not be at one, or the table for 2 is practically on top of an adjacent table. So first thing I do once I get aboard is find out which table I'm assigned to, then go into the dining room and seek it out. If it's not what I asked for, I politely request to have it changed. The earlier you request a table change, the easier it is for them to accommodate you. 

It’s all a matter of preference. If you’re boarding in, say, Florida, and you’ve already been there, then it makes sense to get on the boat as early as possible to start cruising right away.
If you are taking a European cruise, you may spend a couple of days exploring a city before you get on the ship.  Then you may want to board later. 

The earlier you get on, the earlier your vacation begins.

I will always pick the earliest time slot I can find.  The sooner onboard the sooner my actual cruise vacation begins!  I want the full experience and time allowed and the only way I am going to get that is by getting myself onboard…early!

Board early-drag your carry on with you-if using one- until you get access to your room.  Book reservations for shows -the current process-(preCovid it was all booked online), have lunch, utilize drink package, explore ship, use pool?

Board later, put carry on to cabin, your checked  luggage MAY be in the hallway maybe even unpack it,  book reservations, later lunch (Impact dinner?) utilize drink package, maybe explore ship, use pool?
If the pier people are allowing anyone in whenever they want you might have more crowds earlier.
That point is moot if you are suite class (depending on suite and ship) or ,handicapped or The Key, in which case they have a separate line.